1. Field of the Invention
The present invention relates generally to a method and system for objective quality assessment of DCT (discrete cosine transform) based compressed image or video streams, and more particularly pertains to a method and system for objective quality assessment of compressed images of MPEG (moving picture experts group) video streams without the need for the original or reference video source. The method utilizes only the information embedded in the compressed video stream, which consists a sequence of coded frames, to assess the picture visual quality. The inventive method consists of the steps of quantization error estimation, blocking effect estimation, and adaptive combining of these estimates with the motion information if available for each coded frame.
2. Discussion of the Prior Art
Most objective image and video quality measurement approaches require the original image or video source as a reference, which is often not available in practical network image and video communication applications.
In the past several decades, great efforts have been made to develop objective image and video quality measurement approaches that aim to predict perceived image and video quality automatically. The most widely used method is the peak signal-to-noise ratio (PSNR). Other well known approaches include the Sarnoff just-noticeable-difference (JND) model [J. Lubin, “The use of psychophysical data and models in the analysis of display system performance”, in A. B. Watson (ed.), Digital Images and Human Vision, pp. 163–178, MIT press, 1993] and the NASA digital video quality (DVQ) model [A. B. Watson, J. Hu, J. F. McGowan III, and J. B. Mulligan, “Design and performance of a digital video quality metric”, Proceedings of SPIE, Human Vision, Visual Processing, and Digital Display IX, San Jose, 2001]. A novel quality measurement algorithm has also been developed [Z. Wang and L. Lu, “Objective image and video quality assessment,” Technical Report, IBM T J. Watson Research Center, August 2001] which has demonstrated advantages in better correlating with subjective measurement over other approaches in experiments on the video quality experts group (VQEG) Phase I test data set [VQEG, “Final report from the video quality experts group on the validation of objective models of video quality assessment,” http://www.vqeg.org/, March 2000]. These objective quality measurement approaches require the original source sequence as a reference. However, in many practical image and video service applications, especially network image and video communications, the reference source is often not available.
It would be useful to develop single-ended or “blind” video quality measurement algorithms, where access to the reference video sequence is not required.
Little has been done in designing blind video quality assessment methods in the literature. In [M. Knee, “A robust, efficient and accurate single-ended picture quality measure for MPEG-2”, presented at VQEG meeting 2001], a single-ended approach was proposed for quality measurement of MPEG-2 video. This method, known as Picture Appraisal Rating (PAR), assumes a distribution of the DCT coefficient and attempts to predict PSNR using an “activity” parameter and the quantization scale parameters, which are available from the MPEG bitstream. The author claimed that their measurement results have a good correlation with PSNR without using the reference video sequence. Software, hardware and embedded system products that implement this method are for sale by the Snell & Wilcox Company (http://www.snellwilcox.com) The PAR approach has several disadvantages:                It tries to estimate PSNR, which has been widely criticized for not correlating well with perceived video quality. A better benchmarking technique is needed to assess the method and also to improve the method.        Blocking artifact has proved to be one of the most annoying artifacts in Discrete Cosine Transform (DCT) based image and video coding algorithms. However, it is not considered as a distinct factor in PAR. As a result, PAR cannot properly capture and measure the blocking effect, which is similar to PSNR.        There is more information than just quantization scales available from the MPEG bitstream, which is not considered by the PAR method. The statistics of the inverse quantized coefficients and the motion information given by the motion vectors would also be useful for the design of no-reference video quality metrics.        